A known load detecting device detects, using a piezoelectric element, not only a presence of a human body but also abnormality in a human body by extracting a human body-specific vibration component caused by heartbeat, breathing, and the like. For example, JP2000-230853A discloses a load detecting device including a vibration detecting means, a vibration characteristics calculating means, and a load calculating means. The vibration detecting means is arranged on a supporting member for supporting a human body and including a vibration generating portion and a vibration detecting portion provided adjacent to each other. The vibration characteristics calculating means calculates vibration characteristics through propagation of vibration generated by the vibration generating portion on the basis of an output signal from the vibration detecting portion. The load calculating means calculates a load applied to the vibration detecting means based on an output signal from the vibration characteristic calculating means. The vibration generating portion and the vibration detecting portion are both constituted by piezoelectric element. The load detecting device can be installed in a small space with the least trouble accordingly.
According to the load detecting device disclosed, however, since vibration is generated by one piezoelectric element and then the vibration is detected by other piezoelectric element, a piezoelectric member for generating vibration and a piezoelectric member for detecting the vibration are separately required. Thus, in order to measure distribution of load applied to a face of the supporting member and the like, a great number of pair of piezoelectric members for generating and detecting the vibration are necessary, which leads to complication of a structure and increase of a cost of the device.
Thus, a need exists for a load detecting device whose structure remains simple at a time of detection of distribution of load, and cost can be decreased.